A Mach 0.8, 40,000-ft challenge

The 100-in telescope at the heart of NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) is nestled inside the SOFIA 747’s rear fuselage.

The 100-in telescope at the heart of NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) is nestled inside the SOFIA 747’s rear fuselage.

NASA’s Boeing 747SP SOFIA airborne observatory is shown during its second checkout flight in May 2007.

MPC Products, headquartered in Skokie, IL, is completing the final testing phase on a NASAproject to develop an actuator control system that will mechanically operate a cavity door drive system (CDDS) for what is considered to be the largest telescope to ever to be placed in an aircraft.

The reflecting telescope will allow scientists to study distant astronomical objects such as stars, comets, asteroids, forming solar systems, and black holes. It is being permanently installed inside of an airborne astronomical observatory—a modified Boeing 747SP referred to by NASA as SOFIA (Stratospheric Observatory for Infrared Astronomy).

With its 2.5-m aperture, the SOFIA telescope will be capable of making observations that are impossible for even the largest and highest of ground-based telescopes. The telescope, provided to NASA by the DLR (German Aerospace Center), is designed to detect the IR light or energy that is emitted from many different kinds of astronomical objects.

Most forms of IR light/energy are blocked by water vapor in the Earth’s atmosphere, making it almost impossible to view from ground-based telescopes. But flying at about 40,000 ft above ground, the SOFIA telescope will have the capability to detect IR light 100 to 1000 times greater than ground-based telescopes.

One of the key engineering aspects necessary to achieve this observation capability is through the design of the CDDS. The challenge, according to MPC Program Manager Chris Wall, was to design an actuator control system capable of opening and closing the large telescope cavity door on an airplane flying at Mach 0.8—about 500 mph—at 40,000-ft altitude. In addition to speed and altitude, MPC had to take other load factors into consideration, including ice formation, inertial loads, and gravitational forces.

“This has certainly been the most comprehensive software project we’ve undertaken,” Wall said. “Within the next six months, we will be delivering (to NASA) the hardware that will be going on the aircraft to open and close the door system.”

MPC Systems Lead Engineer Matt Polley explained that the telescope is run by a computerized control system, which drives electromagnetic motors to move the telescope into position. The doors are required to follow close to the telescope as it moves, relative to the aircraft maintaining position on the observed object.

“The control system we designed for the doors consists of two redundantly driven actuators commanded by an electronic control unit,” Polley said. “Accurate position and speed control are a critical part of the door design. If the door doesn’t move correctly—if it moves too fast or if it goes beyond the set limits—it could damage the aircraft and cause a catastrophe.”

To develop the actuator control system, MPC’s team designed a test setup to simulate the system’s operational environment using dSPACE tools. A modular system was established using the company’s: DS1005 processor board to achieve real-time and high sampling rates; resolver card; encoder card; and DS2201 analog-to-digital multi-I/O board designed for applications requiring a lot of varying I/O types.

Polley said the dSPACE tools were used to fine-tune the control design and generate a control methodology for simulating the aerodynamic and gravitational loads that the CDDS will encounter during actual operation. More than 400 system-level requirements had to be taken into consideration as part of the design process.

“A crucial element of this project was to simulate the roll and gravitational loads that the actuators will experience during aircraft operation,” Polley said. “Because this behavior could not be defined as a linear function, we had to custom-build a computer system to do closed-loop controls. The system is quite large in terms of the loading, and is more complex than our normal dynamometers, which have lower torque output.”

“It was a major challenge to wrap our hands around the software development aspect,” Polley continued. “We used dSPACE tools to aid in our development process. The tools are very adaptable and are being used on multiple projects here at MPC.”

MPC is wrapping up the final production phase of its first development units. The company is preparing to start a “testing only” phase to prove that its actuator control design works by simulating all conceivable conditions that may be encountered while the telescope and CDDS are in operation and airborne. NASA will obtain the equipment and start testing the CDDS independently in August 2008.

“We have an excellent team in place,” Wall said. “We’ve been working directly with NASA to streamline the software to their expectations. There has been a lot of collaborative effort.”

MPC will be on site to support NASA with the integration of the CDDS and actuator control system onboard the SOFIA aircraft. MPC will also assist NASA as it prepares to engage open-door flight testing, which will result in the first IR pictures of constellations.

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